The hematopoietic stem cell (HSC) is the ultimate progenitor of all types of cells found in the peripheral blood. In addition to being capable of proliferation and differentiation into these cell types, HSC also have the capacity to proliferate without differentiation. This latter property allows a small number of transplanted HSC to repopulate a bone marrow transplant recipient. Our research interest is in how the differentiation and proliferation of HSC are controlled. We have devised methods to purify HSC over 1000 fold, allowing the examination of gene expression in HSC. We have previously shown that the level of mRNA encoding retrovirus receptors in HSC is very low, and that there is a direct correlation between the level of retrovirus receptor mRNA and the frequency with which hematopoietic cell lines or HSC can be transduced. We have identified several populations of HSC expressing higher levels of receptor mRNA. Cord blood HSC which have been cryopreserved have high levels of retrovirus receptor mRNA, and are transduced at 10 fold higher levels that fresh cord blood HSC. In mouse monkey and human studies we have shown that cytokine mobilization increases retrovirus receptor mRNA levels in both peripheral blood and bone marrow HSC, which also correlates with a higher level of transduction. We are currently attempting to induce receptor expression in bone marrow HSC from mice, monkeys and humans. Our ability to purify HSC has allowed us to investigate the molecular biology of stem cells. We have generated several HSC libraries from which we have extracted 3 novel genes, a tyrosine kinase transmembrane receptor, a novel member of the HMG-family of transcription factors, and two novel genes which are homologous to ESTs in the database. Each gene is expressed at high levels in HSC, but is present at low levels in more mature hematopoietic cells. We have isolated full length cDNAs for the first two genes, as well as genomic clones. Polyclonal antibodies against the tyrosine kinase receptor confirm high level expression on HSC. Our current plans are to study the function of the first two genes in both knock out and conventional transgenic mouse models.